With advancements in the field of optics, use of retro-reflectors has become more common. A retro-reflector is defined as a reflector wherein incident rays or radiant energy and reflected rays are parallel for any angle of incidence within the field-of-view. A characteristic of a retro-reflector is that the energy impinging thereon is reflected in a very narrow beam, herein referred to as the retro-reflected beam. This phenomenon was termed retro-reflection. Retro-reflectors are discussed, for instance, in U.S. Pat. No. 6,603,134, which issued Aug. 5, 2003. Those inventors made the discovery that any type of focusing device, in combination with a surface exhibiting any degree of reflectivity and positioned near the focal plane of the device, acts as a retro-reflector.
FIG. 1 is a prior art diagram showing a retro-reflection system containing a lens 20 and a reflective surface 22 positioned in a focal plane 24 of the lens 20. Rays of radiation 26, 28 are directed toward the system, and more particularly toward the lens 20, from a radiation source (not shown). The incident rays in the present illustration are parallel to the optical axis 30 of the lens 20. It should be noted that for the purpose of clarity, the incident rays are shown as being confined to the top half of the lens 20. The incident rays 26 and 28 are refracted by the lens 20 and focused at the focal point 32 of the lens 20, which focal point lies on the reflective surface 22. The rays 26, 28 are then reflected by the reflective surface 22 so that the angle of reflection equals the angle of incidence, and are returned approximately to the lower half of the lens 20 where they are again refracted and emerge therefrom as retro-reflected rays 26R, 28R. The rays 26R and 28R are returned to the radiation source parallel to the incident rays 26, 28 thereof. However, as shown in the drawing, the relative positions of the rays 26, 28 are inverted so that the image returned to the radiation source is also inverted. It should be noted that the lens 20 and reflective surface 22 could, for instance, be elements of an optical device, such as a camera, or an eye of a person or animal.
FIG. 1 shows the basic retro-reflection characteristic of focused lens systems. Rays 26 and 28 enter the aperture, are focused to a point, and then re-projected virtually anti-parallel as shown by rays 26R and 28R. The usual method for calculating the return intensity is to take it as the product of the flux density of the probe radiation and a quantity called the optical augmentation cross section. This is proportional to the aperture area multiplied by the two way transmission and the optical augmentation optical gain. This is defined as the ratio of the aperture area to the image spot area multiplied by the backscatter factor and has dimensions square meters per steradians. Because the focused spot is much smaller than the aperture, this optical gain factor can be very large. This results in a much larger return than would come from the more typical scatterer. The gain in this process is due to the backscatter being narrowly directed, i.e. anti-parallel, rather than diffusively scattered over a hemisphere.
While FIG. 1 depicts the lens 20 positioned at a right angle to the rays 26, 28, U.S. Pat. No. 6,603,134 teaches that retro-reflection does not require an orthogonal relationship between the lens 20 and the rays 26, 28.
One application for the retro-reflectors is to search for cameras, otherwise referred to as optical instruments herein, in an area. FIG. 2 depicts a search system 38 for scanning an area to detect the presence of optical instruments by using retro-reflective properties. The search system 38 includes a scanner 40, including an optical searching device 42, such as a source of infrared light, a detector 44, and a laser 46. The scanner 40 is controlled by a scanning and positioning device 48, which includes a servo motor (not shown). The scanning and positioning device 48 is powered by a power and control device 50, which also supplies power to the scanner 40, and a utilization system 52.
In the operation of the system 38, the scanner 40 is caused to scan a preselected area by means of the scanning and positioning device 48, the scanning and positioning device 48 being programmed by the utilization system 52. The optical searching device 42 emits rays 54, 55. When these rays 54, 55 impinge on an optical instrument 56 exhibiting retro-reflective characteristics, the rays 54, 55 are retro-reflected as retro-reflected rays 54R, 55R and detected by the detector 44. The detector output is then fed to the utilization system 52. The utilization system 52 may be programmed to merely track the optical instrument 56, in which case, the detector output would be fed to the scanning and positioning device 48 and then to the scanner 40, causing the scanning and positioning device 48 to track the optical instrument 56. However, if it is desired to neutralize the optical instrument 56, the utilization system 52 will feed a signal to the laser 46 causing the laser 46 to direct a high intensity laser beam at the optical instrument 56.
This type of search system 38 can have false alarms due to natural and artificial retro-reflections. Natural retro-reflections can occur from sand or other naturally occurring open cube corners. Examples of artificial retro-reflectors include corner cubes such as those used in surveying, rear reflectors on automobiles and bicycles, as well as retro-reflective coated items, such as sportswear and traffic signs.
Thus, a heretofore unaddressed need exists in the industry to address the aforementioned deficiencies and inadequacies.